The present invention generally relates to nuts and in particular to jack-screw nuts, as well as to a tightening device provided with such nuts.
Jack-screw nuts are known in the art. One of such jack screw nuts is disclosed in U.S. Pat. No. 4,622,730. The nut disclosed in this reference has an inner thread connectable with an end of a bolt and a plurality of allen-type screws or hex head screws which are threaded through the wall of the nut and arranged coaxially around the inner thread so as to be equally spaced from one another. The screws extend through threaded holes in the wall of the nut. Their ends are arranged so that when the screws are turned, they abut against a hardened washer sitting on a face of a joint, so as to lift up the nut and to stretch the bolt with a relatively low input force. The idea of this nut is to have each screw of the jack-screw nut carry an equal portion of the created bolt load.
This arrangement has however the following disadvantages. It is necessary to torque down each of the screws onto the washer at a given torque in a given sequence. As the coefficient of friction changes, however the screw which is torqued down creates a side load by tilting the nut. It is therefore very difficult to keep the washer face parallel to the nut face. However this is important for several reasons. If the washer face and the facing nut face are not parallel, one of the screws might carry a larger load than the rest. It either bends or mushrooms out destroying the integrity of the nut. Also, if the washer and the nut face are not parallel to one another, a bending force is applied to the bolt since the nut does not sit completely perpendicular to the bolt axis.